Hydrothermal ageing behaviour of nacre-like zirconia/PMMA and ZTA/PMMA composites for dental applications

Objectives: To evaluate the effect of hydrothermal ageing on a newly developed nacre-like 3 mol% yttria-stabilised tetragonal zirconia polycrystal (3Y-TZP)/ polymethyl methacrylate (PMMA) composite, and to compare this to the zirconia-toughened alumina (ZTA)/PMMA composite. Methods: Hydrothermal ageing with water vapour at different temperatures and time periods was carried out using an autoclave. Materials characterisations were performed using X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). Mechanical testing was also performed to determine flexural strength, elastic modulus, Vickers hardness and fracture toughness. Results: Overall, the ZTA/PMMA composite exhibited improved ageing resistance compared to the 3Y-TZP/PMMA composite. The ZTA/PMMA composite showed slower growth of the monoclinic fraction within the ceramic phase, and a more stable mechanical performance under ageing conditions. However, hydrolysis of the polymer phase during ageing significantly affected microstructural integrity and mechanical properties in both cases. Additionally, the unique brick and mortar architecture within both nacre-like composites appeared to accelerate the ageing process. Significance: The ZTA/PMMA composite was less affected by hydrothermal ageing and is a more promising material for dental use than 3Y-TZP/PMMA composite. In particular the findings of this study highlight the need to improve the ageing resistance of polymer phase and interface, alongside with ageing-resistant ceramics, in order to be able to achieve durable nacre-like composites for dental applications.